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Dust explosion must be controlled but means of necessary vent area design, Vessel strength, also following proper operational procedures and maintaining good housekeeping.
Here a new bag filling plant and silo for plastic manufacturer is designed. So, as a safety advisor the vent sizing for a silo is presented to vent a dust explosion.
Information required for the calculation of the vent sizing are strength of the vessel i.e. silo, explosion properties of the dust, size and shape of the vessel, the static activation pressure that is to open the venting in case of any pressure rise, condition of the dust cloud.
If the dust is found to be toxic then venting should not be done if there's immediate harm to the environment. But in some unavoidable circumstances then the venting is done with an endangered area shall be specified. For that safe discharge area must be calculated to vent the dust to the atmosphere.
And location of venting is chosen on the top of the silo i.e. vertical venting. This assumption made on the condition that the silo is sited in a congested area. Horizontal venting will cause harm the personnel working in the plant area.
Silo Air to bag filter
Vibrating outlets to bag filling stations
Silo is of cylindrical shape.
Diameter = 10m
Height = 30m
Vent duct length = 15 m
Silo Design pressure = 0.25 barg
Material stored in silo is plastic power and also includes Methylene dianiline (MDA).
Here the dust is tested in and 20 litre sphere apparatus to find the maximum rate of pressure rise per unit time. The main apparatus is Sphere explosion vessel, dust dispersion system, ignition source, Pressure monitoring system and control system. This test done as per BS EN 14034-2:2006. And it is found that
(dp/dt) max = 928 bar.s-1
Where (dp/dt) max - Maximum rate of pressure (p) rise per unit time (t)
The objectives of this design are
i. To vent the deflagrating that occurs inside the vessel
ii. To avoid the injury to personnel by vent discharge
iii. To limit the damage of the vessel
iv. To limit the damage of the nearby structures
The following steps are identified for venting sizing with reference to
i. Dust deflagration index Kst must be found,
Kst = (dp/dt)max * V 1/3
Where (dp/dt) max - Maximum rate of pressure (p) rise per unit time (t) =928 bar/s
And volume of the test apparatus is 0.02m3
Kst = 928 * (0.02)1/3 bar.m.s-1
Kst = 252 bar.m.s-1
ii. Now maximum explosion overpressure occurs during dust explosion in an enclosed vessel (non-vented vessel) Pmax which is to determine the explosive characteristics of the dust. Procedure for measuring Pmax is done in 5litre apparatus and the apparatus is designed to withstand an internal overpressure of 20 bar. First required amount of dust is taken for the test. Then the dust is dispersed in the vessel at atmospheric pressure and before that the initial temperature is noted down. Then initial pressure Pi i.e. just a moment before ignition is noted. And the pressure rise recorded as a function of time. And from pressure time curve Pex is determined for the particular dust concentration. And the test is done for various dust concentration and the Pex results are plotted with various dust concentration until the maximum value of Pex is found. And that maximum value is the maximum overpressure Pmax.
This Pmax and Kst plays crucial factor in determining the vent size and design for explosion protection. Here dust mixture composition is not known, so the vent size is based on highest Kst and Pmax value. The result of Pmax for various dust classes is referred from BS EN 14034-2:2006, the table is shown below
Dust explosion class
0 < Kst â‰¤ 200
200 < Kst â‰¤ 300
Kst > 300
St 1- moderate explosible
St 2 - strong explosible
St 3 - very strongly explosible
Hence from the table above Pmax is taken as 10 bar for Kst = 252 bar.m.s-1 and the dust is classified as St 2.
iii. Now the vessel design pressure selection must be measured, if the enclosure vessel is designed as ASTM then Pmawp (Maximum allowable work pressure) can be calculated. Here it is given that design pressure is 0.25barg.
Venting provided should be sufficient to reduce the enclosure vessel rapture due to reduced maximum overpressure, Pred,max
Here Pred,max shall be chose shall not exceed two-third of the vessel strength. Venting shall be provided such that Pred,max shall not exceed the vessel strength to prevent the rapture of vessel during venting.
Pred,max â‰¤ (Pes/ DLF)
Where, DLF - dynamic Load factor as a result of pressure rise. In absence of detailed structural analysis, it is assumed that DLF = 1.5 the design based on weakest structural element.
i.e. Pred,max â‰¤ (2/3)(Pes)
Pes - enclosure strength in bar
Hence, Pred,max = 0.166 bar
iv. Vessel Height to diameter ratio, the ratio of height to diameter of the vessel must be included in determining the vent area. Increase in ratio of height to diameter increases the flame propagation inside the vessel. Hence the estimation of the ratio is given below,
Veff = Ï€ r2 h
Where Veff is the volume of the cylindrical vessel were flame can travel along the path.
h - Height of the cylinder
r - radius of the cylinder
Veff = 3.14 * 5* 5* 30 = 2355m3
Aeff = Veff / H
Where Aeff is the effective area of the cylindrical vessel
Aeff = 2355 / 30 =78.5 m2
Deff = ((4*Aeff)/Ï€)1/2
Where Deff is the effective diameter of the cylindrical vessel
Deff=((4*78.5)/3.14)1/2 = 10m
H/D = 30/10 = 3m
v. Venting cover operation, the following factors to considered for the venting cover operation such as venting opening shall be free and clear, should be obstructed by weather conditions and any dust deposits. The vent cover shall open at its static activation over pressure Pstat. And vent cover should withstand the pressure within the static activation overpressure Pstat.. Here venting cover with specific mass <0.5 kg m-2 is used for resistant free venting and the tolerance range shall not exceed ±25 % as per BS EN 14797:2006. And the venting efficiency is 1 in this case.
And Hence Pstat = 0.2 bar.
Sizing of vent area, here the specific situation must be considered for the venting sizing. Here the material is transferred by pneumatic conveyor. And this is classified as Inhomogeneous dust distribution as per "New findings on explosion venting by R.Siwek".
For vessel length L> 10m
A = 0.0011 * Kst* H *Df * [(1/Dz) (8.6 log Pred,max - 6) - 5.5* log Pred,max + 3.7] ( 1 +1.715 * Pred,max -1.27 * log (H/D))
Where, Df - diameter of the pipeline, here its assumed as 0.1m for effective dust reduction
A - Vent area m2
Dz - effective diameter of the cylindrical vessel
Dz =( (4*v)/Ï€ ) 1/3 = (( 4*2355)/3.14)1/3 = 14
A = 0.0011 * 252 * 30 * 0.1 * [(1/14) (8.6 log 0.166 - 6 ) - 5.5 log 0.166 + 3.7] (1 + 1.715 * 0.3 -1.27 * log 3)
A = 30 m2
Effect of vent ducting, duct is normally to vent the discharge to a safe area away from the work area. But increase in duct length will increase reduced maximum explosion pressure.
P' red,max = -0.03267 * l*(H/D) + 0.3481 * l0.798
Where, l - length of the duct (m)
P' red,max - maximum reduced explosion pressure with vent duct
P' red,max = -0.03267 * 15 * (30/10) + 0.3481 * 150.798
P' red,max = 1.5 bar
P' red,max = 0.2 *(C1 - C2) * (1-(H/D)) + C1
Where C1 = P red,max * (1 + 17.3 *(A*V-0.753)1.6 * l) = 1.027
C2 = (0.0586 * l) + 1.023] * P red,max0.981 - (0.01907 *l) = 0.5
P' red,max = 0.8 bar
And from the above equation relationship between the reduced maximum explosion pressure with vent duct and duct length can be found and also necessary increase in cylindrical vessel strength can also be estimated.
Since the facility is still being designed and from the above result of with effect of vent duct it is evident that reduced maximum explosion pressure increases above the vessel design pressure. So the increase in design pressure and the vessel strength must be re-considered if vent duct is used to deflagrate the flame.
Maximum flame length for dust,
X = Q*V1/3
Where Q - 8 for vertical discharge
X = 8 * (2355)1/3
X = 106.4 m
Maximum flame width,
W = 1.3 * (10*v)1/3 = 37m
Maximum external pressure (dust)
P = 0.2 * Pred,max * A0.1 * V0.18 = 0.188 bar
Where P - maximum external pressure
A - Vent area
V - Volume of the cylindrical vessel.
This bag filling facility handles plastics powder which generates dust must be examined for the explosive characteristics. For that we need to analyse the chemical compositions in it. The explosive dust decomposes generating large enormous energy. This decomposition includes oxygen in the molecule so it is not necessary that it needs air. So it is important to screen the chemical composition first, if the test indicates the presence of explosive characteristics then necessary dust explosion prevention and protection techniques must be implemented as a basis of safety.
In order to prevent the dust explosion following techniques are used
i. Controlling the source of ignition
For explosion protecting the following techniques are used
i. Explosion containment
ii. Explosion suppression
Control of ignition containment
Now the details of each technique are explained in detail below
Controlling the source of ignition, dust explosion cannot occur unless there's a source of ignition. And hence a careful analysis must be done in design, operation and maintenance for the possible sources of ignition. Here are some of possible sources of ignition
a. hot surface
c. electrostatic spark
Flames are one of the sources which can easily ignite the dust. Direct heating i.e. using of burners can be avoided in process where dust generation is possible. Welding works carried on the silo which has possible dust generation inside the vessel. So all hot works carried on silo must be allowed as per the statutory requirements. Any Internal combustion engines near the silo might take in the dust generated nearby and can cause explosion. And hence this combustion engine can be avoided or use of flameproof combustion engines.
Electric power is also one of the sources of ignition. Electric spark which are produced from electrical equipment, if comes in contact with dust will result in explosion. Hence all the electrical equipment must be intrinsically safe and also ATEX 137 EU directive 95/9/EC certified equipment should be used depending upon the dust and zone classification. So this must be done during procurement stage. Once ATEX is implemented then zone classification must be done as a part of ATEX requirement by analysing the possible generation dust from the process i.e. Zone 20 - dust generation is often, zone 21 - dust generation likely to occur or Zone 22 - dust generation not likely to occur.
Dust depositing on hot surface will cause explosion depending on the temperature and geometry of the surface. In most of the cases this can be avoided by good housekeeping. And also Ignition occurs only when the surface temperature reaches the minimum ignition temperature of the dust.
Static electricity is also one of the major hazards in process and chemical industries. When a charged particle comes in contact with the opposite or dissimilar object there will be transfer of charge and will results in spark. Since the powder have charged particle, when it comes in contact with dissimilar particle in transferring or free falling there will be transfer of charge which will generate spark. And the spark generated can cause ignition of the dust. Hence all the metal containers must be earthed so that the charge generated will leak away to the earth. And use non conducting materials are recommended in construction. Below diagram show difference between earthed and non-earthed conductor
Figure 2a: Hazards in non-earthed conductor
Friction is also one of the ways that dust cloud can be ignited. That is when hot particles come in contact with mechanical equipment by rubbing or impacting against the equipment can ignite the dust cloud. This friction ignition depends upon the maximum velocity of the hot particle impacting against the hot surface. And hence use of such mechanical equipment should be avoided.
And the other possible chances of ignition of dust clouds can be through spontaneous combustion. So this type of burning occurs due to self-heating as a result of internal exothermic reaction which is followed by thermal runaway. If this heat release is unable escape will result in ignition. And also sufficient oxygen and dust concentration must be present for the thermal runaway ignition. Hence the safe way is to displace the oxygen is by inerting.
Inerting is a process by sending inert substances to remove or prevent the explosive atmosphere formation. The main objective is to eliminate or to reduce the oxygen level below the lower flammability limit in order to avoid the catastrophic dust explosion, in some case combustion can also occur in very low oxygen level so in that case is safe to replace all the air with inert gases. Even sometimes explosive dust generated inside the vessel can be diluted into non explosive dust by passing certain inert dust e.g. limestone. When inerting there's chance of inert gases gets trapped inside the vessel, where personnel's are accessible for confined space works, this will result in asphyxiation. Hence proper statutory rules must be followed in entry of confined space.
Care should be taken when inert gases are sent into the distribution line. That is before passing the inert gases the impurities such as hazardous substance moisture etc. should be removed from the inert gases by means of filter. And flow of the inert gases must be maintained by the pressure monitoring and controlled. Flow chart for inerting process is shown below
Use of inert dust as an inert medium
Suitable inert gas available e.g.) N2, co2 etc
Performing oxygen limiting measurement at process temperature and pressure
Design dust inert system
Ensure the reliability of the monitoring system
Inert gas cost when compared to other safety technique, are the cost found satisfactory
Consider basis of safety for design and operation
And some the other available prevention techniques include installation of pressure sensor, alarm system in-case of overpressure, Automatic shutdown system in-cases of overpressure, Level indicator, correct operational procedures and Proper maintenance and inspection procedures.
Explosion containment is used to withstand the explosion pressure rise and to prevent the rupture of the containment. The explosion containment usage is accepted when the release of the process materials is not acceptable. First maximum explosion pressure Pmax must be determined, since it is the crucial factor in explosion containment. Hence pressure resistant vessels are designed to withstand the maximum explosion pressure without any deformation or rupturing the vessel. And hence the stress induced by the maximum explosion overpressure should not exceed 50% of the yield strength of the weakest part.
Explosion pressure shock resistant vessel is also designed to handle the maximum explosion pressure but deformation occurs to some extent. And the stress induced by the maximum explosion overpressure should not exceed 90% of the yield strength of the weakest part. Logical flow chart for explosion containment is shown below
Can explosion be contained by knowing Pmax and plant design?
Are the multiple volume is mechanically isolated
Is Rapture of vessel acceptable?
Use pressure shock resistant vessel
Use pressure resistant vessel
Consider basis of safety for design and operation
Cost valid when compared to other safety techniques
Suppression is a technique which identifies the starting point of explosion and extinguishes the growing fire. Normally suppressor is used whenever it is difficult to discharge the pressure and flame in a safe area. Normally it takes 40 - 90ms for an explosion to occur when the dust gets ignited. So now the explosion detector detects the pressure rise in the vessel and it is designed to set the alarm when it reaches the reference pressure rise and activates the suppressor so that it suppresses/extinguishes the growing fire ball inside the vessel. Suppressor can also be used in parallel with venting where sufficient venting area is not achieved. And also it is to noted that explosion detector should withstand to the vibration, shock and resistant against corrosion. Below figure shown is the normal working of suppressor in the vessel.
Normally suppression can be used for a vessel volume up-to 1000m3. For vessel larger than 1000m3 explosion suppression can be used and the explosion suppressor must be within the vessel volume boundary.
Fig 2b: Suppressor working
If a vessel is without suppressor and dust explosion occurs in a enclosed vessel then the pressure rise grow and attains destructible level which is shown below in graph (line A). If a suppressor is installed and suppressor extinguished before the explosion then the maximum pressure rise will be reduced Pred within the maximum vessel design pressure, shown below in graph (line B). In order to achieve the above it also depends upon the suppressor location, suppressor discharge rate and also number of suppressor placed in the vessel.
Graph 2a: Pressure rise with suppressor and without suppressor
There are many types of suppressors available such as hemispherical suppressor, High rate discharge suppressor are used. Normally high rate discharge are most widely used because for their high discharge rate to suppress the fire. In hemispherical suppressor usually liquids i.e. water is used as suppressant and can store upto 5 litres. And the initial velocity of hemispherical suppressor is 200m.s-1 and the discharge time is 10 - 30 ms. For high rate discharge suppressor the suppressant used can be liquid or dry powder. It can suppressant discharge time is within 10 millisecond and suppressant stored upto 40kg.
And the suppressant materials used in order to supress the fire must quench the combustion. And some of the commonly used suppressants are dry powder i.e. dry chemical and water.
Flameless venting is done to vent the explosions without any risk of external flame. Flameless venting device consists of flame arrestor which quenches the flame that propagates outside from the vessel. The main principle is that the arrestor reduces the fuel from flame below the ignition temperature by energy dissipation in the flame arrestor.
Here in this bag filling facility, use of hazardous substance involved. Hence this operation must abide to control of substance hazardous to health Regulation 2002 (As Amended) (COSHH) to control the hazards to the human health.
Here in this bag filling facility there is use of plastic powder which get filled and packed. In this plastic power has an additive known as Methylene dianiline. This plastic powder is a thermoplastic intended for use in injection of moulding machine.
This methylene dianiline is a carcinogen which causes cancer when it is inhaled by the people engaged in bag filling operation. So it is necessary to conduct control of substance hazardous to health risk assessment. And to evaluate the allowable exposure limits and the necessary measure to be taken while handing the hazardous materials.
Main steps to be followed to prevent the health hazards and to comply with COSHH are as follows
i. Determine the risk
ii. Control measure implementation
iii. Control the exposure
iv. Continual improvement and practice of the control measure implemented
v. Monitoring the exposure level with the control measures
vi. Providing health monitoring check up
vii. Prepare Emergency plan and conduct emergency mock drill
viii. Providing training and necessary information to the employees
Substances or chemicals that are hazardous to personnel health will come under COSHH. Here methylene dianiline is used which is identified as a potential carcinogen and hence the operation should comply with COSHH to control the health hazards and improve the operation.
Determine the hazard
The first step is to identify the hazard whether the substance used in the process causes health hazard to employee engaged in work. Here it is identified that Methylene Dianiline is a potential carcinogen. So operators engaged for bag filling, sealing and engaged in cleaning activities are at risk if exposed. So first we need to find the possible exposure points. From analysing the operation involved in bag filling facility. The possible release/exposure points are identified below,
i. filling arms in bag filling station building - it vibrates to prevent clogging
ii. Opening the valve fast will cause sudden release of pressure.
iii. Bag sealing - possible dust generation since the bag is left opened
iv. Cleaning the spilled dust near filling area
v. Pneumatic conveyor - possible leak point will cause dust discharge
Here it is identified that possibility of the substance route to affect operator health is through inhalation when released in air.
And hazards of methylene dianiline and its chemical properties are taken from CHIP classification, Now CHIP regulation gradually replaced by European CLP. And the hazard classification is taken from European regulation EC No 1271/2008 on Classification labelling and Packing (CLP) from Table 3.2 part3 of Annexure I to directive 67/548/EEC
International Chemical Identification
Crac Cat 2;R 45 Mutta.Cat,3; R 68
Classification is taken from European regulation EC No 1271/2008 on Classification labelling and Packing (CLP) from Table 3.1 part3 of Annexure VI to directive 67/548/EEC
International Chemical Identification
Hazardous Class and Category codes
Hazardous Statement Codes
Pictogram signal word code
Hazardous Statement Codes
Suppl. Hazardous Statement Codes
STOT SE 1
STOT RE 2
Skin sens 1
Where R - Risk phase and H - Hazard , Classification of levels of danger i.e. harmful, toxic, very toxic as per CHIP regulation. Here MDA is classified as potential carcinogen R 45 - Cancer causing substance.
Deciding proper safe guarding measure
Since here the plant is in designing stage so the possible release/exposure points, population exposed to the hazardous substance and route of entry are identified and necessary control measure are indentified below to implement from the designing stage. So COSHH essential uses out of the risk assessment information it chooses one of the methods for control measure shown below,
The factors used in identifying appropriate controls measure are in below figure,
So the following steps are followed in identifying adequate control measure as mentioned in "COSHH essentials: Easy steps to control chemicals"
i. Group the hazards identified
ii. Grouping the physical properties of the amount used
iii. Asses the anticipated exposure
iv. Now combine step 1 to 3 to form a generic assessment
Grouping the hazard, hazards are classified between A to E by R-Phase given in CHIP and H-Phase given in CLP. Below the table shows the classification of hazard group. In the below table units, mg/m3 - milligrams per cubic meter and ppm - parts per million. From below table methylene dialine classified under group E dust.
Now to determine the predictive exposure we must first classify the hazardous substance physical properties. Here in bag filling operation, hazardous substance is in dust form, since the plastic powders are granule will generate dust. So as per COSHH essential they have presented a table for identifying the determinants of the hazardous substance. That is the factor for deciding the physical properties for solid are dustiness and for the liquid is the volatility.
And based upon the below shown table here methylene dianiline is identified as fine solid and light power and the corresponding determinant is identified as high.
And after identifying the determinant and amount used as per COSHH essential has identified four band of exposure potential and the table is shown below,
Here in bag filling and packing operation the main product is plastic powder which contains methylene dianiline as an additive. So the quantities used which assumed to be in tonnes, the main aim of this plant is packing plastic powder. And the exposure predictor band here it is identified as EP 4.
sNow to decide which the control approach is adequate enough to control the situation of health hazard has to be identified from the range given which is used in COSHH essential. And table is shown below
And based upon the above control approval table and exposure predictor table COSHH essential formed a table relating exposure predictor to control approach. And the table is shown below.
In-order to choose the type of control measure recommended we have to relate the target airborne exposure to the exposure predictor band .Hence for this bag filling facility type-4 control is recommended because the concentration level of the dust is unknown.
Sample COSHH Risk assessment
What are the hazard
What will harm and who?
What are you doing
Breathing in dust from filling station
Since the dust contains MDA might cause cancer and irritates the respiratory system
Get cab and filtered air supply
Conveyor to silo, to filling station
Check for leaks weekly
Bagging plastic powder
Storage and dispatch
Get cab and filtered air supply
Cleaning the plastic powder dust spill
Vacuum hose to dust extraction
Changing dust filter
Use of P3 respirator
Contract out this work
Examination & Test - COSHH
Instruction and training
Here to reduce the possible exposures to hazardous substance below are the following recommendations,
i. Minimizing the generation of plastic dust such as designing conveying system in such a way to reduce the impact with hard surface to reduce the dust generation i.e. use of long sweep elbows.
ii. Minimising the release of plastic dust such as keep silo in good conditions i.e. avoid crack, proper maintenance etc., maintain the transfer equipment in good seal condition to avoid leaks.
iii. Plastic dust can be captured and contained.
iv. Create awareness among the employee about the hazards associated in handling hazardous substance and use of MSDS.
v. Regular health surveillance must be conducted to employee exposed to risk.
vi. Use of proper respiratory PPE's while handling with plastic powder. As per COSHH essential it is identified suitable PPE's for the selected group hazard. Table is shown below and it is identified as Assigned protection factor 200. This APF is in reference with BS 4275
The new bag filling line and silo is being constructed for a plastic manufacturer which is located in congested area which means the silo is located in between the nearby structures and objects but the whole plant is located in plain and partly terrain area. This facility involves transfer of plastic powder from the plant to silo for storage so that it can used to store plastic powder prior to the bagging and distribution. Here in the silo there is possible release of dust into the atmosphere due to overpressure or overfilling. Since the dust generated inside the silo is vented to atmosphere so it must meet to the current environment legislation in order to avoid air pollution. From Silo the plastic powder is sent to bag filling station. The bag filling station comprises a building in which there are four bag filling stations. Hence an environmental impact aspect must be undertaken before the commencement stage .
Silo Air to bag filter
Vibrating outlets to bag filling stations